What is liquefaction?

Liquefaction is when earthquake shaking causes some soils to behave more like a liquid than a soil. The shaking causes water pressure between soil particles to rise so that sand and silt is suspended in the ground water.

Sand and silt being thrown out of the ground is a good sign of liquefaction on your site – but absence doesn’t prove it hasn’t occurred below the ground.

Lateral spreading

This is when liquefaction isn’t contained to one area and spreads – usually because it’s near a ‘free-face’ such as a riverbank or open channel.

Future land performance

Even if your land and property wasn’t affected by liquefaction due to the latest earthquake events, this doesn’t mean that it won’t be in future.

We’ll assess what has happened, but will also provide you an indication of how your land might behave in future events

What sort of foundation repair will I need?

What types of foundation repairs are there?

Foundations relevel

A foundation relevel can be undertaken in various ways and will depend on your foundation type.

This will generally involve either ‘packing’ foundation elements by inserting wedges above them to lift the floor to the required height; or by ‘uplifting’ from beneath the foundation using mechanical methods (hydraulic jacks or screw piles) or grout (resin or low mobility grout).

Full foundation replacement

For a full foundation replacement the house is entirely removed from the site, lifted above its current position, or propped in place, and the foundations are demolished.

New foundations will be constructed in accordance with current guidance and the house put back onto the replacement foundation.

New foundations

Shallow piles

Piles are used to transmit load from the building and its contents to the ground – shallow piles are usually used in ‘good ground’ (as defined in NZS 3604) and generally are founded at shallow depths (<1.2 m).

Enhanced slabs

Used for rebuilding on TC2 damaged land – this concrete slab foundation system is designed to cope with predicted future land damage for TC2 sites.

Surface structure foundations

Surface structures can be used on most TC3 sites without ground improvement or deep foundation works. They adjust to varying levels of lateral spread and can be re-levelled in the event of future differential settlements caused by earthquakes.

Deep piles

These give you dependable vertical load capacity during predicted earthquakes – but aren’t suitable for all dwellings as they need to be founded in dense sand or gravel.

Ground improvement

Many options exist – the choice depends on site specific criteria such as existing ground conditions, liquefaction susceptibility, size of the site, neighbouring properties, groundwater and cost.

Ground improvement allows you to build a shallow foundation, such as a concrete slab, without the concern of your land performing poorly during an earthquake.

What if my property is in the Port Hills?

The process is very much the same as with flat land, however liquefaction damage is a low risk.

It’s important you understand your property’s level of risk when it comes to slope stability and other potential geological hazard such as boulder roll, cliff collapse and mass land movement. Armed with the right information gained from a ‘site specific’ investigation, these challenges can be appropriately assessed and managed.